1. Field of the Invention
The present invention relates to a cooling system structure for a vehicular water-cooled internal combustion engine. The internal combustion engine includes a cylinder section constituting, together with a crankcase for rotatably supporting a crankshaft, an engine main unit, with a cylinder body that rises upwardly from the crankcase. A piston is connected to the crankshaft and is slidably fitted within a cylinder head connected to the cylinder body. Water jackets are formed in the cylinder body and the cylinder head, respectively. A radiator is disposed forwardly of the engine main unit with a water pump disposed on the engine main unit. The water pump performs a forced circulation of coolant through the radiator and the water jackets.
The present invention also relates generally to power units for small sized vehicles. The power unit is mounted on a vehicle body frame including an internal combustion engine and a transmission. The internal combustion engine includes a crankcase that rotatably supports a crankshaft having an axis extending in a vehicle width direction and a cylinder body in which a piston connected to the crankshaft is slidably fitted and that rises upwardly from a front portion of the crankcase. The cylinder body includes a water jacket formed therein for circulating coolant. The transmission is disposed rearwardly of the crankshaft and is housed in a transmission housing section formed in the crankcase rearwardly of the cylinder body. The present invention relates particularly to an improved cooling system structure in a power unit for use in a small sized vehicle.
2. Description of Background Art
A known arrangement, as disclosed in Japanese Patent No. 4842715 wherein a cooling system structure includes a water pump disposed at a lower portion on a first end side of an engine main unit in a direction extending along an axis of a crankshaft. A thermostat is disposed at a front portion of a cylinder head on a side opposite to the water pump in the direction extending along the axis of the crankshaft. The arrangement further makes a discharge port of the water pump communicating directly with a front lower portion of a water jacket formed in a cylinder body, thereby allowing coolant to flow to the side of the thermostat by way of the water jacket in the cylinder body and a water jacket in the cylinder head.
A radiator needs to be disposed at a position wherein the radiator easily receives airflow forwardly of an engine main unit. In the arrangement disclosed in Japanese Patent No. 4842715, external pipes disposed on both side of the engine main unit connect between the thermostat and the radiator, and between the radiator and the water pump. A need thus exists for improvements in terms of appearance.
Japanese Patent Laid-Open No. 2005-9350 discloses a disposition of a water pump in a power unit mounted on a small sized vehicle, such as a motorcycle. More specifically, the water pump is disposed on a side wall of a cylinder body wherein the positioning is less likely to be subject to restrictions in terms of layout and an extending of the piping length is avoided.
In a small sized vehicle, such as a motorcycle, in which the position at which the power unit is mounted greatly affects an interior space of an occupant, a need exists for minimizing the size of the power unit in a vehicle width direction as much as possible. However, in Japanese Patent Laid-Open No. 2005-9350 the water pump that is operatively associated and connected with a first end of a crankshaft is disposed so as to bulge outwardly in the vehicle width direction from the side wall of the cylinder body. This invites a larger size of the power unit in the vehicle width direction and a longer piping length between a water jacket in the cylinder body and the water pump.